1. Field of the Invention
The present invention relates to a ZnO--Pd composite catalyst capable of eliminating hazardous components from the air, such as CO and NOx.
2. Description of the Prior Art
Examples of hazardous gases which enter or are generated in an automobile include CO, NOx and sulfur compounds (such as H.sub.2 S and SOx). Pd is known as a catalyst effective in eliminating these components, especially in oxidizing and decomposing CO, at a normal temperature. Japanese Patent Publication (JPA) No. 1-234729 teaches an air cleaner comprising a photocatalyst supported on an adsorbent for decomposing odor components and oxidizing NOx adsorbed by the adsorbent, when the photocatalyst is irradiated by a light beam.
Among platinum group catalysts which are effective in eliminating CO by its oxidation, Pd has a catalytic activity higher than that of Pt at normal temperature. However, sulfur compounds (such as H.sub.2 S and SOx) coexisting in the air to be treated would serve as a catalyst poison to Pd, so that when an air filter with Pd catalyst is used for purifying the air containing the sulfur compounds, Pd would be poisoned by the sulfur compounds to gradually degrade its catalytic activity. Pt is not susceptible to poisoning by the sulfur compounds and, therefore, has been mainly used in the prior art air filters. However, Pt has a lower catalyst activity and is much more expensive than Pd.